The mall had just closed Sunday evening when the exercise began. The drill took shoppers still in the vicinity by surprise.

Police agencies often hold active shooter exercises in schools, but this is the first time they have practiced in Colorado in a mall setting.

“We know that we have intruders that are armed that are in the mall in the food court,” said Dave Joly, spokesman for the regional office of the Federal Bureau of Investigations, FBI. “And we have people who are in there — again this is an exercise — but we have people who are injured, people who have been shot, and we have law enforcement responding to that as well.”

The scenario is designed to test the officers’ ability to deal with multiple challenges including several shooters, possible hostages and explosive devices. The exercise was headed by the FBI, but included the Broomfield Police Department and North Metro Fire Rescue. Each agency brought everything they have to the situation, including sharpshooters, SWAT teams and mobile command units.

“This is a very real world exercise. It’s very fluid. We don’t really know how the scenario is going to play out that will change as our command staff make decisions reacting to this particular incident,” Joly explained.

It is so real world that none of the mall patrons knew about the exercise until an announcement was made on the property.

“We were like, ‘What is going on?’ ” said one shopper.

“I mean I would have liked to know before I show up,” said another shopper.

That was by design. Mall officials purposely didn’t notify the public.

“We knew this would be the best time to host something like this with our partners. And, yeah, see how this all plays out. And try to be as real-life as possible,” said Heather Drake, Flatiron Crossing spokesperson.

There were monitors watching the exercise play out. All the agencies involved will be graded on how well they did. Joly says it’s an opportunity for the agencies to work together and learn what areas they can improve upon. Active shooter exercises like this one are playing out in 53 cities across the country.